A fuel cell conventionally includes a stack of elementary cells, within which an electrochemical reaction occurs between two reagents which are introduced continuously. The fuel, such as hydrogen for fuel cells operating with hydrogen/oxygen mixtures (PEMFC) or methanol for fuel cells operating with methanol/oxygen mixtures (DMFC), is brought into contact with the anode, while the oxidizer, generally oxygen, is brought into contact with the cathode. The anode and the cathode are separated by an electrolyte, of the ion exchange membrane type. The electrochemical reaction, the energy of which is converted into electric energy, is split into two half-reactions:                oxidation of the fuel, occurring at the anode/electrolyte interface producing in the case of hydrogen fuel cells, protons H+, which will cross the electrolyte towards the cathode, and electrons which return to the outer circuit, in order to contribute to the production of electric energy;        reduction of the oxidizer, occurring at the electrolyte/cathode interface with production of water in the case of hydrogen fuel cells.        
The electrochemical reaction occurs at an electrode-membrane-electrode assembly.
The electrode-membrane-electrode assembly is a very thin assembly with a thickness of the order of one millimeter and each electrode is supplied with fuel and oxidizing gases for example by means of a splined plate, a so-called bipolar plate.
The ion conducting membrane is generally an organic membrane comprising ionic groups which, in the presence of water, allow conduction of the protons produced at the anode by oxidation of hydrogen.
More specifically, in an aqueous medium, the acid groups borne by the membranes totally dissociate and release free protons, which are surrounded with one or several water molecules, thereby ensuring transport of protons according to a carrier mechanism ensured by the hydration water. The mobility of the protons in the membrane is therefore closely related to the water content (i.e., in other words, to the swellability of the membrane) and to the conductivity of the membrane (related to the number of acid sites of the latter).
In addition to the capability of ensuring proton conduction, the membranes also have to meet the following specificities:                low permeability to gases (notably to H2 gas for PEMFC fuel cells and to methanol vapor for DMFC fuel cells, in order to ensure a good seal between the anode and cathode compartments of the cell as well as maximum electric and catalytic efficiency;        sufficient absorption of water in order to promote a good swelling rate, in order to ensure good transport of protons from dissociation of acid protons, thereby forming a hydrated ionic phase in the totality of the volume of the membrane;        good electrochemical and mechanical stability, notably reaction innocuousness toward reactive gases (such as hydrogen or methanol vapors) and resistance to gas pressures to which the cell is subject.        
In order to attempt to overcome such requirements, the first polymers to have been elaborated are polymers having protogenic groups of the sulfonic acid type, more particularly sulfonic perfluorinated polymers such as Nafion®. These polymers have good proton transfer by a strong acidity related to the sulfonic acid groups and by a large hydration number λ (λ>15, being defined as the number of water molecules per protogenic group). However, the membranes elaborated from these polymers only have actual effectiveness for large hydration levels and are thus limited to uses at low temperatures (i.e., temperatures below 90° C.) and with high relative humidity (for example of more than 80%).
Alternatives other than Nafion® have been proposed for forming membranes for fuel cells.
Thus, some authors have proposed membranes based on polymers comprising nitrogen-containing heterocyclic groups, these groups allowing proton conduction said to be by proton jumps between heterocyclic groups (as described in J. Membr. Sci. 2001, 185, 29-39).
In order to operate, these membranes do not absolutely require the presence of an aqueous solution. It should also be noted that proton dissociation of the aforementioned heterocyclic groups is very low (these groups having a pKa greater than that of water), which, for obtaining effective conductivity requires the adjunction in the membrane, in addition to the polymers, of one or several dopants forming an additional source for providing protons.
Other authors have proposed membranes based on polymers comprising protogenic groups of the phosphonic acid type with however the drawbacks inherent to these groups, i.e.:                lower acidity of the phosphonic acid groups than that of sulfonic acid groups, which requires a level of presence of these groups in the polymers, greater than that of the sulfonic acid groups in order to obtain an equivalent conduction;        difficulties for synthesizing this type of polymer.        
These polymers bearing phosphonic acid groups may be obtained via two synthesis routes:                either by polymerization of monomers bearing phosphonate groups followed by hydrolysis of the phosphonate groups into phosphonic acid groups;        or by introducing phosphonate groups into existing polymers followed by hydrolysis of the phosphonate groups into phosphonic acid groups.        
As regards the first route, it is not very used, since no suitable monomers comprising phosphonate groups exist on the market. In order to apply it, it is thus necessary to make said monomers often at the expense of several non-trivial synthesis steps, which may be expensive to apply because of the high cost of the reagents.
As regards the second route, copolymers were designed from base copolymers comprising aromatic groups. Thus, Laffite and Jannasch (J. Polym. Sci., Part A: Poly. Chem. 2007, 45, 269-283) designed copolymers bearing phosphonic groups starting with a base copolymer comprising aromatic groups bearing sulfone groups, and by subjecting it to a lithiation step and a phosphonation step by cross-coupling, the sulfone groups being thereby replaced with phosphonate groups followed by hydrolysis for transforming them into phosphonic groups. The synthesis of these copolymers however requires significant safety conditions because of the use of butyllithium. Furthermore, the resulting copolymers have a high glassy transition temperature, which makes it difficult to shape them into the form of membranes. Finally, these copolymers have a proton conductivity of about 5 mS/cm at 100° C. for relative humidities of less than 30% and with an ion exchange capacity ranging from 0.6 to 1.8 mequiv./g.
The inventors propose the development of novel copolymers which may be used for forming fuel cell membranes, which meet the following requirements:                adjustable proton conductivity, which may be high for temperatures ranging from room temperature up to 150° C. and for relative humidities of less than 50%;        thermal stability at high temperatures, for example ranging up to 150° C.;        facility of being shaped as a membrane, notably related to the ability of these copolymers of being solubilized in organic solvents, such as dimethylsulfoxide;        facilitated synthesis of these copolymers.        